In developmental processes of optical networks, various signal frames have been standardized in terms of signal speed. In a synchronous optical network/synchronous digital hierarchy (SONET/SDH) method disclosed in Non Patent Literature 1, STM (synchronous transport module)-1 for accommodating a 150-Mbps signal has been used as a standard, and subsequently, STM-4 (600 Mbps), STM-16 (2.4 Gbps), and STM-64 (10 Gbps) signals each corresponding to a transmission speed increasing four-fold have been used as standards. The SONET/SDH method is configured so that a payload area for containing information to be transmitted and a transmission information header are added to data having a fixed frame length. Those signals are mainly exchanged as electrical signals, and in that case, signals at the same hierarchy are exchanged. In that case, in SONET/SDH, clocks used for data transmission are synchronized among all nodes, to thereby prevent inconsistency in the data speed from occurring between the signals to be exchanged.
On the other hand, an optical transport network (OTN) method disclosed in Non Patent Literature 2 for adding error correction information in order to improve a signal-to-noise ratio tolerance in an optical transmission line has become mainstream in recent years. In FIGS. 9A and 9B, frame structures used in the OTN method are illustrated. An optical-channel data unit (ODU) frame is illustrated in FIG. 9A, and an optical-channel transport unit (OTU) frame is illustrated in FIG. 9B. In the OTN method, an OTU is defined so as to share a header area with an optical-channel data unit (ODU) header area of an ODU, which is obtained by adding the transmission information header to a payload area (optical-channel payload unit; OPU) for containing the information to be transmitted, and further so as to have added thereto (forward) error correction parity information (forward error correction; FEC). “OH” (overhead) represents control information (maintenance overhead), and “FA” (frame alignment) represents frame alignment.
In the OTN method, transmission rates such as ODU1/OTU1 (2.5 Gbps), ODU2/OTU2 (10 Gbps), ODU3/OTU3 (40 Gbps), and ODU4/OTU4 (100 Gbps) are currently defined based on their transmission speeds.
In the optical network, in order to allow those signals to coexist, a ladder structure is designed so as to allow each transmission frame to accommodate transmission frames at lower-level speeds in multiple hierarchies. For example, STM (synchronous transport module)-4 can accommodate four STM-1 signals, and an OTU4 signal can accommodate ten OTU2 signals.
FIG. 10 is a diagram for illustrating an example of a case where OTU3 accommodates an ODU2 signal. The payload area (OPU3) of OTU3 is formed by using four ODTU (optical channel data tributary units) 23's or sixteen ODTU3.ts's. In the former case, ODTU23s and OTU2s are allocated to each other on a one-to-one basis, and in the latter case, four signals (TSs) obtained by demultiplexing the ODU2 signal are allocated to four ODTU3.ts's as illustrated in the figure.
Those signals have the same frame length and frame structure at all the hierarchies, but the transmission speed itself of the signal is not an integral multiple of the transmission speed of a lower-level hierarchy so that a higher-level hierarchy signal (HO-ODU) allows a header part of a lower-level hierarchy signal (LO-ODU) to be transmitted as well. Further, in the OTN, the respective optical paths for transmission as LO-ODUk and HO-OTUj operate within a scope that does not depart from a jitter specification described in G.709, and hence even at the same hierarchy, clocks thereof are not always synchronized.
In recent years, the transmission speed has reached a speed as high as 100 Gbps. Therefore, there is a tendency that devices that exchange those signals increase in circuit scale. Further, even when a low-speed signal is subjected to parallelization in order to undergo the signal transmission within the device, parallel signal transmission itself increases in speed in order to prevent the device itself from increasing in scale, which causes a problem in skew compensation between parallel signals. In addition, various signal speeds are mixed in a situation in which transmission frames having a fixed length are continuously transmitted, and hence there is a demand for a device configuration ensuring consistent clock of those signals.
According to the configuration presented in Patent Literature 1, an LO-ODUk signal to be exchanged is divided in units of ODTU4.ts of a device system, and asynchronously accommodated in a common frame operating in accordance with a clock common to the device system. The signal exchange is realized for a device by cross-connection of a common frame group based on a switch of a time-division method operating in accordance with the clock common to the device system. At a time of being multiplexed in an HO-ODUj signal output from a signal exchange device, the cross-connected signal is subjected to signal transfer from the common frame onto ODTUjk or ODTUj.ts of the HO-ODUj signal. In this case, the HO-ODUj signal differs in the transmission rate depending on a value of j as described above, and hence the signal transfer is transfer using an asynchronous accommodation circuit.
With the method of Patent Literature 1, while the signal exchange in units of ODTU4.ts is allowed for input/output of various HO-ODUj signals, it is necessary for realization thereof to provide an accommodation circuit for an asynchronous signal for time-division cross-connection before and after the cross-connection, and it is necessary for realization of time division to establish frame synchronization at a cross-connection unit. In addition, there is a demand for an advanced circuit implementation technology such as necessity for such a design as to establish synchronization between common frames having the same LO-ODUk signal at an accommodating unit of HO-ODUj of an exchange destination, or unification of interfaces for the cross-connection.
Aside from those problems with the signal exchange device, an optical transmission capacity per optical path has been increasing on a current optical network, and there are a few cases where a transmission capacity for HO-ODUj to be transmitted is not used up, with the result that unused ODTU areas are scattered. It is possible to attain a more efficient transmission capacity in the optical network by using those unused areas, but when the capacity of the optical path to be used for transmission is larger than the capacity of each of fragmented paths, those fragmented paths cannot be used. When a signal is divided into a plurality of paths, there is a problem of providing a method of absorbing skew that occurs among paths due to an asynchronous operation clock and a difference in transmission delay among the optical paths.